Inherit default branch name for subgroups

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/293953

**Problem**

We fetch default branch name from the direct project's group. But we
ignore default branch settings from the root group when the project's
group is a subgroup.

**Solution**

Use root group default branch name settings if the direct project's
group doesn't redefine them.

app/models/concerns/has_repository.rb

@@ -77,9 +77,14 @@ module HasRepository
   def default_branch_from_preferences
     return unless empty_repo?
 
-    group_branch_default_name = group&.default_branch_name if respond_to?(:group)
+    (default_branch_from_group_preferences || Gitlab::CurrentSettings.default_branch_name).presence
+  end
+
+  def default_branch_from_group_preferences
+    return unless respond_to?(:group)
+    return unless group
 
-    (group_branch_default_name || Gitlab::CurrentSettings.default_branch_name).presence
+    group.default_branch_name || group.root_ancestor.default_branch_name
   end
 
   def reload_default_branch

changelogs/unreleased/293953_inherit_default_branch_name_for_subgroups.yml

+---
+title: Inherit default branch name for subgroups
+merge_request: 57101
+author:
+type: fixed

spec/models/project_spec.rb

@@ -5041,57 +5041,27 @@ RSpec.describe Project, factory_default: :keep do
   end
 
   describe '#default_branch' do
-    context 'with an empty repository' do
-      let_it_be(:project) { create(:project_empty_repo) }
+    context 'with default_branch_name' do
+      let_it_be_with_refind(:root_group) { create(:group) }
+      let_it_be_with_refind(:project_group) { create(:group, parent: root_group) }
+      let_it_be_with_refind(:project) { create(:project, path: 'avatar', namespace: project_group) }
 
-      context 'group.default_branch_name is available' do
-        let(:project_group) { create(:group) }
-        let(:project) { create(:project, path: 'avatar', namespace: project_group) }
-
-        before do
-          expect(Gitlab::CurrentSettings)
-            .not_to receive(:default_branch_name)
-
-          expect(project.group)
-            .to receive(:default_branch_name)
-            .and_return('example_branch')
-        end
-
-        it 'returns the group default value' do
-          expect(project.default_branch).to eq('example_branch')
-        end
+      where(:instance_branch, :root_group_branch, :project_group_branch, :project_branch) do
+        ''      | nil           | nil            | nil
+        nil     | nil           | nil            | nil
+        'main'  | nil           | nil            | 'main'
+        'main'  | 'root_branch' | nil            | 'root_branch'
+        'main'  | 'root_branch' | 'group_branch' | 'group_branch'
       end
 
-      context 'Gitlab::CurrentSettings.default_branch_name is available' do
+      with_them do
         before do
-          expect(Gitlab::CurrentSettings)
-            .to receive(:default_branch_name)
-            .and_return(example_branch_name)
-        end
-
-        context 'is missing or nil' do
-          let(:example_branch_name) { nil }
-
-          it "returns nil" do
-            expect(project.default_branch).to be_nil
-          end
+          allow(Gitlab::CurrentSettings).to receive(:default_branch_name).and_return(instance_branch)
+          root_group.namespace_settings.update!(default_branch_name: root_group_branch)
+          project_group.namespace_settings.update!(default_branch_name: project_group_branch)
         end
 
-        context 'is blank' do
-          let(:example_branch_name) { '' }
-
-          it 'returns nil' do
-            expect(project.default_branch).to be_nil
-          end
-        end
-
-        context 'is present' do
-          let(:example_branch_name) { 'example_branch_name' }
-
-          it 'returns the expected branch name' do
-            expect(project.default_branch).to eq(example_branch_name)
-          end
-        end
+        it { expect(project.default_branch).to eq(project_branch) }
       end
     end
   end